Sim works in my old phone but not in the iphone?

I'm getting my dads iPhone and I have a sim on a different contract when I put it in the iPhone it doesn't come up even though the iPhone is unlocked from the previous network it used

Depending on how old the sim is sometimes new phones don't register them,
Also, this may sound silly but is a common mistake, make sure you're putting the sim in the phone the right way around

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